Myogenin and desmin immunohistochemistry in the assessment of post-chemotherapy genitourinary embryonal rhabdomyosarcoma: prognostic and management implications.	PURPOSE: Posttreatment genitourinary embryonal rhabdomyosarcoma often shows well differentiated rhabdomyoblasts, which are detectable on routine histological staining. Definite areas of residual undifferentiated rhabdomyosarcoma indicate residual/recurrent disease. However, the recent use of immunohistochemical staining with desmin and myogenin in resected specimens and surveillance biopsies following adjuvant therapy may demonstrate scant positive staining cells that appear undifferentiated on light microscopy. To our knowledge the clinical significance of this finding is currently unknown. Therefore, we reviewed our retrospective experience with genitourinary embryonal rhabdomyosarcoma to examine the relationship between immunostain positive undifferentiated cells and subsequent clinical outcome. MATERIALS AND METHODS: A total of 14 children with a median age of 2.75 years (range 8 months to 7 years) with genitourinary embryonal rhabdomyosarcoma were identified in the histopathology database. All had biopsy confirmation of the diagnosis, followed by multi-agent chemotherapy. Two children in whom there was obvious residual active tumor at the resection margins were excluded from further analysis. Histopathological findings in all patients on the resection/posttreatment biopsy were reviewed. All specimens were immunostained with desmin and myogenin to detect residual undifferentiated rhabdomyoblasts. The relation between histopathological findings and outcome was determined. RESULTS: There were 14 cases of genitourinary embryonal rhabdomyosarcoma. In 2 cases (14%) residual embryonal tumor was pathologically confirmed following initial treatment. In 12 cases no obvious residual tumor was present following initial therapy. Rhabdomyosarcoma affected the bladder in 10 cases and the vagina in 2. There were no distant metastases in any child. Ten patients underwent local resection following chemotherapy and 2 underwent followup biopsies only without resection. A total of 11 cases showed well differentiated, posttreatment rhabdomyoblasts that was identifiable on routine hematoxylin and eosin staining with margins apparently free of tumor and 1 showed no morphological evidence of residual rhabdomyosarcoma. However, all cases demonstrated at least scant abnormal desmin and myogenin positive cells in the specimens. Four patients had no further treatment and none had clinical recurrence. All were well 10 years (range 8 to 13) after treatment. Eight patients received further treatment (chemotherapy and/or radiotherapy) based on clinical and pathological findings, followed by further resection in 3. One patient died of disease but 7 were well a median of 7.2 years (range 8 months to 13 years) after treatment. CONCLUSIONS: The significance of undifferentiated myogenin/desmin positive cells in genitourinary embryonal rhabdomyosarcoma in the absence of morphological residual/recurrent embryonal rhabdomyosarcoma remains unclear since such cells can be detected in all cases of posttreatment embryonal rhabdomyosarcoma. In some cases findings are associated with clinical disease recurrence, while others with identical histopathological findings following initial treatment have no clinical sequelae even in the absence of further treatment. In genitourinary embryonal rhabdomyosarcoma close and regular clinical surveillance is essential. Desmin/myogenin immunohistochemistry to detect scattered undifferentiated cells does not appear to provide useful prognostic information.	bladder